Thermionic tube amplifier



May 23, 1933. w. LA v. CARLSON 1,910,239

THERMIONIC TUBE AMPLIFIER Filed March ll, 1951 USCILLATUR INVENTOR.

Wendell L.. Carlson,

/1' A TTORNE Y.

Patented May 23, 1933 UNTEB STATES PATsfar ortica WENDELL LA VERNE CARLSON, or HADDONFIELD, NEwJEasEY,Ass1GNoR, BY MnsNn AssisNnnN'rs, 'ro RADIO cORPonATON or AMER-ICA, on New YORK, N. Y., A con.-

PORATION OF DELAWARE THERMINIC TUBE .ANIPLIFIER Application filed March 11, 1931.

My invention relatesto thermionic tube amplifiers and it has particular relation to amplifiers wherein thermionic tubes of the screen-grid type are used. Y

rhe screen-grid tube is capable of giving enormous amplification of weak signals. It is, therefore, especially useful in radio receivers of the so-called midget type where space is at a premium and where the greatest amplification with the smallest number of tubes is necessary.

By reason of the interposition of a screengrid between the control grid and the plate, the grid-plate capacity of the screen-grid tube is greatly reduced but it is not entirely eliminated. If the impedance of the external plate circuit of the tube is raised, therefore, to approximately the internal plateimpedance of the tube, in order to obtain high amplification, oscillations will be generated provided both plate and grid circuits are tuned to substantially the same .fre-

quency.

ln addition to the requirement, in midget receivers, for miximumamplification, the apparatus must be so disposed and interconnected that the greatest selectivity, compatible with the necessary amplification, may' be had from a limited number' of tubes. Selectivity, however, can best be obtained through the use of coupling circuits wherein the L/C ratio is low and the amplification obtainable, when such circuits are exclusively used, is insufiicient.

lt is, accordingly, an object of my invention to provide a thermionic tube amplifier that shall, with a limited number of tubes, embody the best compromise between amplification, selectivity, fidelity and stability.

According to my invention I so design and proportion the circuits inter-connecting successive thermionic tubes in an amplifier that, when the input and output circuits of any given tube are resonant to the same frequency, the L/C ratios thereof are relatively low and when the input andoutput circuits of a tube are resonant to different frequencies their respective L/ C ratios are relatively high.

The novel features that I consider charac- Serial No. 521,789.

teristic of my invention are set forth with particularity in the appended claims. The invention in its entirety, however, both as to its organization and its method of operation, together with additional objects .and advantages thereof, will best be understood from the following description of a specific embodiment, when read in connection with the accompanying drawing.

The single figure of the drawing is a diagrammatic view of a portion of a thermionic tube amplifier including a preferred embodiment of my invention.

The apparatus illustrated in the drawing comprises a first detector vtube 1, an intermediate frequency amplifier tube 3 and a second detector tube 5, the first two tubes mentioned being of therscreen-grid type while the second detector tube is of the ordinary three-electrode type. The complete receiver comprises an oscillator 7 including a thermionic tube (not shown) and also a stage of audio frequency amplification and a loudspeaker. lnasmuch, however, as my invention is particularly related to the Vfirst detector, the intermediate frequency amplifier and the second detector of the system, the other incidental apparatus, since it is perfectly familiar to those skilled in the art, has not been illustrated.

The several tubes have been shown in the drawing as being provided with separate C and B batteries 9 and 11 respectively, although, in a commercial receiver, all potentials are supplied from a power transformer, the primary winding of which may be connected to any availabie (S0-cycle distribution network and which has a plurality of secondary windings for providing filament and heater potentials and for energizing a rectifying device'that supplies biasing and plate potentials.

The first detector tube has an input circuit constituted by an inductor 13 and a condenser 15 for tuning it to the carrier frequency of an incoming signal.

The first detector and the intermediate frequency amplifier are inter-coupled through a transformer 17 lhaving a tunable primary winding 19 and a tunable secondary winding 21, the secondary winding constituting the input circuit for the intermediate frequency amplifier. The primary winding has a very high L/C ratio and the impedance thereof approximates the plate-impedance of the first detector tube. he secondary winding has a low L/C ratio.

The intermediate frequency amplifier is coupled to the second detector by'way of a transformer 23 which is similar in substan# tially all respects to the first described transformer except that the primary and secondary windings are equivalentY in characteristics to the secondary and primary windings of the said first mentioned transformer, respectively. In other words, the output circuit of the intermediate frequency amplifier is constituted by ka tunable transformer primary winding 25 having a low L/C ratio, while the input circuit of the second detector tube is constituted'by a tunable secondary winding 27 having a high L/C ratio.

It will be apparent from a consideration of the drawing and of the foregoing description that the input and Ioutput circuits of the first detector tube, although the output circuit has very high impedances, are not tuned to the same frequency. Conditions favoring regeneration, therefore, are not present and high amplification is obtained without resulting instability.

Insofar as the intermediate frequency amplifier' stage is concerned, the input and output circuits are each tuned to the same frequency but since the L/C ratio in both circuits is low, it also does not tend to oscillate. The low L/C ratio, furthermore, increases markedly the selectivity obtainable.

The output circuit of the second Ydetector tube includes the primary 'winding 29 'of an audio frequency transform-er 3l and 'a pass condenser 33. This circuit hashigh impedance to audio frequencies but 1t is not resonant to any frequency within the tuning range of the input circuit of the said tube. Conditions for regeneration, are, therefore, not present in the second detector tube.

In a preferred `commercial embodiment of my invention the primary winding 'of the transformer between the first detector tube and the intermediate frequency amplifier tube has Lan inductance of 19 milli'h'enries, and the secondary windingan inductance of 411/2 millihenries, the L/C ratio of the primary circuit being 362 X106 when it is tuned to the intermediate frequency of 175 kilocycles 'by the shuntV condenser and the L/C ratio of the secondary circuit, when tuned to the Asame frequency, being 22.4 106.

The transformer between the second and third tubes is the same, mechanically considered, as that between the first and second tubes.

It will be apparent from the foregoing description of an amplifier constructed according to my invention that the stage having low L/C input and output circuits contrib utes selectivity to the system and that the lessened amplification therein is compensated by the stages preceding and following it, which stages are provided with input and output circuits having high L/C ratios.

My invention, therefore, is highly advantageous in that it permits an excellentY compromise between amplification, selectivity and stability in amplifier systems of the type wherein tuned'inter-tube transformers are used.

Although I have shown and described a specific embodiment of my invention, many variations thereof will be apparent to those skilled in the art to which it pertains. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spi-rit of the appended claims.

I claim as my invention:

l. In combination, a plurality of thermionic tubes inter-coupled in cascade, the coupling means between one tube and a preceding and a succeeding tube bein-g constituted by transformers having tunable primary and secondary windings, and the L/C ratio of the primary winding in the output circuit of said one tube being lower than that of the secondary winding associated therewith, and the L/C ratio of the secondary winding in the input circ-nit of said one tube being lower than that of the primary winding associated therewith, and both windings being tunedto substantially the same predetermined frequency.

2. In combination, a plurality of thermionic tubes inter-coupled in cascade, the coupling means between rone tube and a preceding tube being -constituted by :a transI- former having tunable primary and secondlary windings, the L/C ratio of the'primary being greater than that of the secondary at any frequency in the tuning range, and the coupling means between the said one tube and a. succeeding tube being constituted by a transformer having a primary winding the L/C ratio of which is less than that of the secondary.

3. In an amplifier at least three ther- -mioni'c tubes disposed in cascade, a transformer coupling the first and second tubes and 'a transformer coupling the second and third tubes, the L/C ratio of the primary winding of the first transformer being substantially the same yas the L/'C ratio of the secondary winding of the second transformer.

4. In 'an amplifier, at least three thermionic tubes disposed in cascade, a transformer coupling the first and second tubes and a transformer coupling the second 'and third tubes, the L/C ratio of the secondary winding of the first transformer being substantially the same as the L/C ratio of the primary winding of the second transformer.

5. The combination as set forth in claim 3 wherein the first two tubes are of the screen-grid type.

6. The combination as set forth in claim 3 wherein the first two tubes are of the screengrid type and the third tube is of the threeelcctrode type.

7. T he lcombination as set forth in claim 4 wherein the first two tubes are of the screengrid type.

8. T he combination as set forth in claim 4 wherein the first two tubes are of the screengrid type and the third tube is of the three-` electrode type.

9. In a cascade amplifier, a thermionic amplier stage including input and output circuits tuned to substantially the same signal frequency, a tuning inductance in each of said circuits, the L/C ratios of said inductances being relatively low, and an inductance associated with each of said firstnamed inductances to provide a transformer, said last named inductances each having a relatively high L/C ratio.

10. In an amplifier, a therinionic amplifier stage having input and output circuits tuned te substantially the same frequency and each including a tuning inductance having a relatively low L/C ratio, a second thermionic amplifier stage preceding said first named stage and having input and output circuits resonant to different frequencies, each of said circuits including a tuning inductance, the L/C ratio of which is relatively high, and a third thermionic amplifier stage succeeding said rst named stage and having an input circuit including a tuning inductance, the L/C ratio of which is relatively high.

11. In combination, a plurality of therniionic tubes having input and output circuits intercoupled to connect said tubes in cascade relation to each other, each of said circuits including a tuning inductance and being tuned to predetermined frequencies, the input and the output circuits of one of said tubes being tunable for operation at substantially the same frequency and the inductances therein having relatively low L/ C ratios, and the input and output circuitsof certain others of said tubes being tunable for operation at different frequencies and the inductances therein having relatively high L/C ratios.

12. In a thermionic amplifier system, a thermionic amplifier tube having an input circuit and an output circuit each tunable to operate at substantially the same frequency, a transformer Winding providing a tuning inductance in each of said circuits, said induetances each having a relatively low L/O ratio, and transformer windings associated with said first named windings providing inductances having relatively high L/C ratios and being tuned to substantially the same frequency as the first named windings.

13. In a superheterodyne receiver having a therinionic first detector device, a therlmionic intermediate frequency amplifier device and a thermionic second detector device, each having an input circuit and an output circuit, transformer coupling means interposed between said circuits to connect said devices in cascade relation to each other, n

the input circuit of the second detector de-` vice to the same signal frequency as the input and output circuits of the intermediate frequency amplifier device, said circuits including transformer windings providing inductances having relatively high L/ C ratios.

14. An amplifier having a plurality of amplifier stages each comprising an electric discharge device having a grid input circuit and an anode output circuit, characterized by the fact that said input and output circuits in one of said stages are responsive to signal currents of substantially the same frequency and include inductance elements having a low L/C ratio, and that said circuits in another of said stages are responsive to signal currents differing in frequency and include inductance elements having a high Ii/C ratio.

In testimony whereof, I have hereunto subscribed my name this 4th day of March 1931.

WENDELL LA VERNE CARLSON. 

